1. Field of the Invention
The present invention relates to a sheet forming unit of a paper machine, in particular a tissue machine for producing a fibrous web. The sheet forming unit includes a headbox which outputs a suspension jet and has an inner fabric and an outer fabric, which in each case forms a loop. A forming element is arranged within the loop of the inner fabric, and guide rolls and a white-water box for receiving the white water which accumulates in the region of the forming element are arranged within the loop of the outer fabric. The inner fabric and the outer fabric form a gap for receiving the suspension jet and jointly wrap around the forming element at least partially in a wraparound region. The present invention also relates to a method for operating a sheet forming unit.
2. Description of the Related Art
In generic sheet forming units in the current state of the art, solutions are known for efficient white water removal. Document WO 82/02910 describes solutions for twin wire sections as well as for twin wire sections with an upstream Fourdrinier wire section. The white water which is thrown off by the forming cylinder is directed over guide walls into a catch pan. In this process, the thrown off white water experiences only a slight loss in velocity, so that only small flow cross sections are necessary for removal of the white water. For this reason, mixing of this thrown off white water with other water components is avoided.
With ever increasing speeds, contamination due to water mists containing fines occur increasingly in known sheet forming units, especially in the region of the outer fabric. In order to avoid impairment of the production and quality of the produced paper web, the water mist is removed through pipe lines and hoods through active vacuum extraction by means of blowers. This solution is expensive in regard to the required equipment and energy consumption.
What is needed in the art is a cost effective solution for avoiding the disruptive effects of contamination in the region of the sheet forming unit.